Recent years have seen CRTs being replaced by display panels contained in display panel modules (electronic devices). The latter are typically liquid crystal panels because of their numerous advantages, such as low power consumption and compactness.
However, a liquid crystal panel still costs 10 times a CRT. To create a greater market for the liquid crystal panel, it is essential to cut costs for the liquid crystal panel and its peripherals.
The conventional liquid crystal driver chip (semiconductor chip) driving a liquid crystal panel is packaged on a flexible substrate which is an insulating base member provided with a wiring layer thereon. The package chip, or semiconductor device, is connected to an edge of a liquid crystal panel.
Packaging methods for the semiconductor device, by which the semiconductor chip is packaged on the flexible substrate, include COF (chip on FPC (flexible print circuit)) and TCP (tape carrier package).
In a TCP, the base member of the flexible substrate has holes (device holes) for mounting a semiconductor chip. The semiconductor chip is connected at contact terminals on its electrodes to inner leads/wires which stick out from the device holes. In contrast, for COF, the flexible substrate has no device holes. The inner leads, connected to a semiconductor chip, are formed on the base member.
Today's focus is on COF methods because it is easy to reduce the width of the semiconductor device. Narrow-width devices well meet demands for reductions in the frame size of display panel modules in which the semiconductor device is disposed in the frame.
The inner leads stand on the base member in COF, whereas in TCP, the inner leads stick out from the device holes for connection to the semiconductor chip. COF therefore more readily accommodate reductions in the width of the semiconductor device than TCP.
FIG. 4 illustrates an exemplary liquid crystal panel module in which a semiconductor device is packaged. In the figure, 41 indicates the liquid crystal panel. Along an edge of the liquid crystal panel 41 are provided COF semiconductor devices 44 in which the chip is packaged by a COF method. The devices 44 are connected to the panel 41 via anisotropic conducting film, or ACF, for example. Each semiconductor device 44 contains a flexible substrate and a liquid crystal driver chip (semiconductor chip) 45 mounted on the substrate.
The flexible substrate of the semiconductor device 44 has external contact terminals formed thereon: i.e., output outer leads 42 and input outer leads 43. The semiconductor device 44 is connected to the liquid crystal panel 41 through the output outer leads 42 and to a circuit board 46 through the input outer leads 43. The semiconductor device 44, connected to the liquid crystal panel 41, exchanges signals and turns power on/off through the circuit board 46.
The liquid crystal driver chip 45 feeds analog signals to the liquid crystal panel 41 through the flexible substrate in the semiconductor device 44. The analog signals travel from the liquid crystal driver chip 45 via the flexible substrate to the liquid crystal panel 41. On the liquid crystal panel 41, those contact terminals for connection to the flexible substrate usually align parallel to an edge of the liquid crystal panel 41. Therefore, the output outer leads 42 of the flexible substrate, to which the contact terminals of the liquid crystal panel 41 are connected, are similarly arranged.
From these design points of view, the semiconductor device can be best reduced in width by: aligning the output terminals of the liquid crystal driver chip 45 along a long side of the liquid crystal driver chip 45 so that the terminals are parallel to the length of the liquid crystal driver chip 45 and straightening the wires between the output terminals of the liquid crystal driver chip 45 and the flexible substrate of the output outer leads 42 to the extent possible. The same modifications are required with the input terminals of the liquid crystal driver chip 45 to render the semiconductor device narrow and long. The input terminals are in many cases also arranged along a long side of the liquid crystal driver chip 45.
As a result, the conventional liquid crystal driver chip 45 naturally has a very high external aspect ratio with the short sides extremely shorter than the long sides.
There is another demand to reduce the size of the liquid crystal driver chip 45 as much as possible. The objective of doing so is to reduce the cost of the semiconductor device.
Apart from these demands for narrower, longer semiconductor devices and smaller liquid crystal driver chips, a notable trend is happening recently. Traditional line-reversal drive schemes for liquid crystal in a liquid crystal panel module are mostly replaced by dot-reversal drive schemes. Liquid crystal driver chips which drive source signal lines are designed for dot-reversal schemes.
A liquid crystal driver chip 45 for source signal lines needs nine grayscale power supply terminals to achieve a 64 grayscale level display, for example. The dot-reversal scheme would need double that number, that is, 18 grayscale power supply terminals, because the scheme utilizes both positive and negative reference grayscale levels.
Another technology trend as noteworthy as the dot-reversal scheme is RSDS (reduced swing differential signaling) for liquid crystal driver chips. RSDS retains low noise levels for digital signals in the liquid crystal driver chip 45 until analog signals are output to the liquid crystal panel 41. RSDS is based on differentials on two signal lines and needs double the number of signal lines for traditional single signal wiring.
With these mainstream dot-reversal driving and RSDS technologies, the liquid crystal driver chip 45 now has 40 or more input signal lines. As for the output signal lines, the liquid crystal driver chip 45 has 1024×3 (R, G, B)=3072 of them for an XGA (1024×768) liquid crystal panel 41, for example. To drive the panel with eight liquid crystal driver chips, each liquid crystal driver chip 15 drives 384 source signal lines. Each liquid crystal driver chip for source signal lines has 384 output signal lines.
Now, let us consider how long the liquid crystal driver chip 45 should be on its long sides, one for output and the other for input. If 384 outputs are needed with 50-μm pitches, for example, the long side for output where output terminals will be formed needs be at least 384×0.05=19.2 mm long. On the other hand, if 45 inputs are needed with 75-μm pitches, the long side for input where input terminals will be formed needs be no longer than 3.375 mm, less than one-fifth for the long-side for output.
Ideally, the output and input terminals should be separated: the output terminals should be on the long side for output opposite the output outer leads connected to the liquid crystal panel 41, and the input terminals on the long side for input opposite the input outer leads connected to the circuit board 46. This design however would allow the output terminals, which outnumber the input terminals by far, to place constraints on the long sides of the liquid crystal driver chip 54, which in turn would restrict the downscaling of the chip for lower cost.
In view of these circumstances, News Release, No. 2001-103 (made public on Dec. 11, 2001) by Sharp Co., Ltd. among other publications suggests to place some of the output terminals along the long side for input of the liquid crystal driver chip to provide the long side for output and the long side for input with an equal number of electrodes, thereby minimizing the length of the long side of the liquid crystal driver chip.
FIG. 5 illustrates the COF semiconductor device described in News Release, No. 2001-103 in which some of the output terminals are placed along the long side for input where the input terminals of the liquid crystal driver chip are arranged. FIG. 5 is presented as a plan view so as to clearly show differences between the device and the present invention. In the figure, the semiconductor device 61, along with others (not shown), is yet to be individually punched off the long and narrow tape carrier 50. They are still lined end to end on the tape carrier 50. The liquid crystal driver chip 54 and the solder resist 53 giving protection to wires 52 on the flexible substrate 51 are depicted as being transparent so as to visualize how the wires 52 are routed.
Referring to FIG. 5, in the semiconductor device 61, the liquid crystal driver chip 54 sitting on the flexible substrate 51 has some of its output terminals formed, as indicated by Xs, on the long side for input which is opposite the side where input outer leads 57 are provided. Those of the wires 52 on the flexible substrate 51 which are connected to the liquid crystal driver chip 54 at the output terminals on the long side for input (indicated by 52c) are routed to turn 180° from the inner leads 55 toward the output outer leads 56.
Those of the wires 52 on the flexible substrate 51 other than the ones indicated by 52c are formed linearly. Specifically, those indicated by 52a which are connected to the input terminals on the long side for input of the liquid crystal driver chip 54 are formed to run linearly to the input outer leads 57. In addition, those indicated by 52b which are connected to the output terminals on the long side for output of the liquid crystal driver chip 54 opposite the side where the output outer leads 56 are provided are formed to run linearly to the output outer leads 56.
As mentioned earlier, ideally, wires on the flexible substrate connected to the liquid crystal driver chip should run linearly to the input outer leads if they are intended to be coupled to the input terminals and linearly to the output outer leads if they are intended to be coupled to the output terminals.
However, as shown in FIG. 5, in this conventional design of the liquid crystal driver chip 54 in which the input terminals and the output terminals X are both placed on the same long side of the liquid crystal driver chip 54 for the purpose of size reduction, the wires 52c connected to the output terminals X on the long side for input travel around the liquid crystal driver chip 54 by making a 180° turn before ultimately reaching the output outer leads 56.
The wire extending from the output terminal X which is nearest the corner of the liquid crystal driver chip 54 takes the shortest path because it has to skirt only the liquid crystal driver chip 54. Meanwhile, the wire extending from an adjacent terminal X has to skirt not only the liquid crystal driver chip 54, but the wire connected to the output terminal X nearest the corner. Further, the wire extending from a next output terminal X has to skirt these two wires too. The more the output terminals X formed on the input side, the more wires each wire has to travel around in addition to the liquid crystal driver chip 54, and the longer path it has to take.
These paths in the conventional design require increasingly more space to accommodate the wires 52c on the flexible substrate 51, adding to the size of the flexible substrate 51. The increased size of the flexible substrate 51 would be an obstacle in the efficient use of the tape carrier. This could kill off the cost savings on the semiconductor device achieved by the downscaling of the semiconductor chip (liquid crystal driver chip).
Further, the design involves many curves to make the 180° turn. Curves are not desirable for etching which is a method typically used for patterning on the flexible substrate 51, because etching often cannot sufficiently remove materials from curves. Resultant reduced yields might also lower the cost saving benefits.